Apparatus for stretch drawing sheet stock under controlled tension

ABSTRACT

An apparatus for stretch drawing sheet metal between complementary male and female dies arranged between two stretch heads which grip opposite margins of the sheet and tension it unidimensionally into its yield range independently of die closure. While maintaining the sheet so tensioned with its lateral margins unconfined, the heads wrap the sheet about the male die prior to closure of the dies. Beginning at the end of the wrapping operation and continuing during final closure of the dies, the gripping pressure exerted by the heads on the gripped margins of the sheet is reduced sufficiently to eliminate the independent tensioning by the heads while holding the sheet steady. The reduction is such as to permit the margins to slip between the gripping jaws of the head under the tensioning force imposed by closure of the dies at some portions along the width of the sheet whereat die configuration requires that additional stock exteriorly of the dies at the ends thereof be drawn or fed in between the dies, while other portions where additional stock is not required are held firmly in place but free from the independent tensioning by the heads. The final drawing by the dies occurs before the sheet can work harden as a result of having been independently tensioned into the yield range.

United States Patent 72 Inventor Robert A. MacKende Chagrin Falls, Ohio [21 1 Appl. No. 790,519

[22] Filed Jan. 13. 1969 [45] Patented Aug. 10, 1971 [73 Assignee The Cyril Bath Company Cleveland, Olio [54] APPARATUS FOR STRETCH DRAWING SHEET STOCK UNDER CONTROLLED TENSION RelerenoesCited UNITED STATES PATENTS 12/1965 Dolney 7/1969 Mackenzie 11/1960 Bath 1/1963 Wheeler Primary Examiner-Charles W. Lanham Assistant Examiner-Michael J. Keenan Attorney-John Harrow Leonard ABSTRACT: An apparatus for stretch drawing sheet metal between complementary male and female dies arranged between two stretch heads which grip opposite margins of the sheet and tension it unidimensionally into its yield range independently of die closure.

While maintaining the sheet so tensioned with its lateral margins unconfined, the heads wrap the sheet about the male die prior to closure of the dies. Beginning at the end of the wrapping operation and continuing during final closure of the dies, the gripping pressure exerted by the heads on the gripped margins of the sheet is reduced suificiently to eliminate the independent tensioning by the heads while holding the sheet steady. The reduction is such as to permit the margins to slip between the gripping jaws of the head under the tensioning force imposed by closure of the dies at some portions along the width of the sheet whereat die configuration requires that additional stock exteriorly of the dies at the ends thereof be drawn or fed in between the dies, while other portions where additional stock is not required are held firmly in place but free from the independent tensioning by the heads.

The final drawing by the dies occurs before the sheet can work harden as a result of having been independently tensioned into the yield range.

Patented Aug. 10, 1971 6 Sheets-Sheet 1 INVENTOR. W,

*4 ATTORNEY.

Patented Aug. l0, 1911- 3,597,955

6 She ets-Sheet 2 INVENTOR.

fATTORNEY.

Patentd Aug. 10,1971

6 Sheets-Sheet 5 INV NTOR. QM fl-W- BY ATTORNEY.

Patented Aug. 10,1971 3,597,955

6 Sheets-Sheet 4.

QMa-M 41" ATTORNEY INVENTOIF.

Patented Aing. 10, 1971-v I 3,597,955

6 Sheets-Sheet 6 J as 39 I l9 7 2O 83 1 2| r 44 INVENTOR. I

*F- .6. BM #7 2 3 ATTORNEY.

APPARATUS FOR STRETCH DRAWING SHEET STOCK UNDER CONTROLLED TENSION This-invention relates to a new and improved prestretch fixture and combination thereof with a drawing die press, and is an improvement on the inventions disclosed in US. Letters Pat. ofCyril J. Bath, No. 2,961,028, issued Nov. 22, 1960, and the pending application of Robert A. Mackenzie, Ser. No. 646,084, filed June 14, 1967, now U.S. Pat. No. 3,452,573, entitled Stretch Wrapping Fixture and Combination Thereof with Drawing Dies and Press.

US. Pat. No. 2,961,028 discloses the stretch drawing of sheet metal stock between companion drawing dies while the stock is held under tension independently of the dies by tension applying gripping heads which engage the opposite end margins of the sheet between clamping jaws and tension the sheet unidimensionally into its yield range and wrap it about the male die while maintaining it in said range. It teaches that in certain instances, the independent tensioning of the sheet unidimensionally by the gripper heads can be reduced so that the total of the independent tension applied by the gripper heads and that occasioned by closure of the dies maintain the sheet within the yield range until final closure of the dies. This it accomplishes by reducing the yieldable hydraulic pressure which drives the stretch heads on the stretching strokes and resists their return.

In the above application, the gripper heads are mounted on rigid links and swing in fixed paths so chosen that the heads tension the sheet into the yield range and maintain it in that range independently of die closure while wrapping the stock onto the male die and until the dies are fully closed.

It was found that in some instances, due to the configuration of the dies, more elongation of the metal was required at some portions along the width of the sheet than at other portions. However, since the stretch heads are of fixed shape, the stretch necessary to tension into the yield range the sheet portions requiring the lesser elongation to fit the dies would result in overstretching of the sheet portions requiring greater elongation to fit the dies.

. Contrarily, if the stretch necessary to tension the sheet portions requiring the greater elongation were made controlling to prevent overstretching at such portions, the portions requiring less elongation would be insufficiently stretched.

The sheet is unidimensionally tensioned by the heads inde- I pendently of die closure into the yield range, as heretofore,

Again, it sometimes happens that localized bulges or concavities in the dies necessitate different total amounts of the elongated stretched metal aligned in the stretching direction with such bulges and concavities.

The optimum would be that each increment of width of the sheet be tensioned to the same degree of yield and have the same unit elongation throughout its length as the other increments.

In accordance with the above patent, the independent tensioning force of the heads was superimposed on the tensioning force imposed by die configuration and closure until the completion ofthe die-drawing operation.

In accordance with the above application, the objectionable variation in tensioning and elongation at different increments of width of the sheet was ameliorated by bending the sheet by extraneous power means about the convex cross section of the male die and more nearly into conformance with that cross section during stretch wrapping on the male die so that the lateral margins of the sheet were brought under more nearly the same tensioning force and elongation as the midportion of the sheet which had to be wrapped around the downward sloping end portion of the male die.

In the present invention, the stretch heads are provided with jaws which are operated by direct acting means, such as direct-acting hydraulic piston and .cylinder assemblages, so that the clamping pressure of the jaws on the end margins of the sheet can be varied at will and accurately controlled in relation to die operation.

and fully wrapped about the male die, the stretch heads reaching their final wrapping position prior to closure of the dies and maintaining their independent tensioning force on the sheet. However, during final closure of the dies, and while the heads remain in final position, the gripping pressure is reduced so that the independent tensioning force of the heads ceases, and additional elongation of the stock due to die closure is reduced to a minimum, the additional metal at the ends of the dies being drawn into the dies to supply the extra stock due to the gripped margins of the sheet slipping toward the dies from between the gripping jaws. Thus the independent tension applied by the gripping heads is not superposed on the imposed by the dies on the metal, and overstretching is eliminated, the dies drawing in metal only where needed while the heads impose only slight resistance to draw in along the entire margin. Thus, the metal is first wrapped fully about the male die while under independently applied tension in the yield range, and after completion of the wrapping operation and during closure of the dies, the independent tension on the sheet is reduced so that metal can be drawn between the jaws at any increment of width of the gripped margin to the degree necessary for forming of the workpiece without overstretching any portion thereof.

This operation is enhanced to a substantial extent due to the closure of the dies being effected immediately after the stretch heads have completed the wrapping operation, and while the metal is still in the more readily flowable state due to its having been independently tensioned into the yield range by the gripping heads and before work hardening resulting from such tension has set in. Insofar as any portion of the sheet is maintained under tension in the yield range at this point, such is due to forces imposed by the dies, the heads merely holding the metal so that the sheet as a whole does not shift its position in the dies during final closure.

Various specific objects and advantages will become apparent from the following description wherein reference is made to the drawings in which:

FIG. 1 is a front elevation of a drawing die press with the prestretch fixtures of the present invention mounted therein;

FIG. 2 is an enlarged plan view of the prestretch fixtures illustrated in FIG. 1;

FIG. 3 is a front elevation of the structure illustrated in FIG.

FIG. 4 is a vertical cross-sectional view of the fixture illustrated in FIGS. 2 and 3, and is taken on the line 4-4 in FIG. 3, part thereof being shown in elevation;

FIG. 5 is a hydraulic diagram showing the hydraulic pressure fluid applications to the fixture during the loading operation of the fixture; and

FIG. 6 is a diagram similar to FIG. 5 showing the application of the hydraulic pressure fluid to the fixture during the operation of the stretch draw fixture.

Referring to FIG. 1 of the drawings, the fixture is sho m as installed in a conventional upright hydraulic press 1, it being understood that it may be used in any of the conventional die drawing presses. The press 1, illustrated, has a bed 2 with upright guide members 3 at the comers thereof guiding a ram 6a are shown for convenience in illustrating one of the features of the present invention. The dies 5 and 6, when closed, define a concavo-convex pattern both transversely and longitudinally with the general convex face of the pattern facing upwardly, but with indentations or joggles therein, both longitudinally and transversely, resulting in reverse curvature at portions of the pattern.

The stock is tensioned by stretch fixtures, along one dimension only. Conventional holddown devices are not employed, and the lateral margins of the sheet are free to be drawn in toward the dies.

The ram 4 is driven on its forming and return strokes by means of hydraulic piston and cylinder assemblages 7, including cylinders 8 and pistons 9, each having a piston rod 10 secured to the ram. If desired, of course, the ordinary cranktype drawing die press may be employed.

The stretch fixture of the present invention, indicated generally at 12, is mounted on the bed 2 of the press.

The fixture comprises a frame having a baseplate 13 and front and rear upright side frame members 14, reinforced by suitable ribs 15. Disposed between the upright side frame member 14 is a bed plate or bolster 16 on which the die 5 is supported. Stretch heads 17 and 45 are arranged at opposite ends of the die space of the bolster 16.

At one end of the bolster a stretch head 17 is arranged, this stretch head being in the form of a rigid C-shaped beam extending forwardly and rearwardly of the die space of the press and carrying a stationary gripping jaw 18. Carried by the head, is a row of hydraulic cylinders '19 in which pistons 20 are reciprocable. The pistons are provided with rods 21 which engage a common rigid clamping jaw carrier 22 on which is mounted a jaw 23 coextensive endwise with the jaw 18.

The stretch head is mounted in fixed position on a link 24 which comprises a rigid box structure 25 having at each end a block 26 with an internally threaded bore 26a. Jackscrews 27 are threadably mounted in the bores 26a so as to fon-n an adjustable rigid extension of the box 25. Each block 26 may be split, as indicated at 28, and provided with a locking screw 29, so that after adjustment of the jackscrew 27 in the box 25 it can be locked securely in the adjusted position by a bolt 29.

Since the linkage supporting the box 25 at the front of the press is duplicated at the rear of the press, only that linkage in connection with the front will be discussed in detail.

At its upper or outer end, the jackscrew 27 is secured in fixed position to a rigid rocker arm 30 which is mounted on the frame of the fixture for rocking about a horizontal pivot 31 extending forwardly and rearwardly of the press. The link is so chosen that by swinging it, its rocker arm 30, and head 17 as a unit about the axis of the pivot 31, the head 17 can be disposed in a loading position above the level of the top of the die 5 or, if it is desired to load the sheet in an arched position to prevent sagging in the center, the stretch head 17 may be raised to a position below the position above the die 5, but one in which the jaws open upwardly at a substantial angle so as to receive the downwardly sloping end margins of the arched sheet.

The head 17 can be swung from this loading position to a final lowered position, such as illustrated in FIG. 3, in which the stock has been wrapped fully a out the male die.

The die 5 is positioned on the bed 16 of the fixture so that its crest is as near as practical at the axis of the pivot 31. This is to assure that during the stretch forming operation about the male die the metal sheet, while tensioned into the yield range, is bent about the crest of the male die 5 rather than being stretched across the crest. This assures a clear and fixed crest in the finished part.

In order to swing the links 24 and 30, and thereby the stretch head 17, about the axes of the pivot 31, holddown piston and cylinder assemblages 35 are provided. In each assemblage, a cylinder 36 is mounted on trunnions 37 on the lower end portion of the link 24 so as to rock about an axis parallel to the pivot 31. A suitable reinforced frame 38, which is a boxlike structure, is mounted on the box 25 of the link 24 for supporting the outboard end of the trunnion 37. Each cylinder carries a piston 39, having a piston rod 40 which is connected to a suitable tongue 41. The tongue 41 is pivotally connected by a pivot 42 between upright supporting plates 43 mounted on the baseplate 13.

Thus, upon operation of the assemblages 35 to fully extended position, the stretch head 17 is swung upwardly to loading position, and upon retraction of the pistons 39, the link is swung counterclockwise downwardly, thus swinging the head downwardly about the axis of the pivots 31.

correspondingly, at the opposite end of the bolster 16, or die support, is a stretch head 45 mounted on a link 46 having at each end jackscrew extension 47, the head 45 and link jackscrews 47 corresponding to the head 17, link 24 and jackscrews 27 heretofore described. The head 45 is provided with direct acting clamping hydraulic cylinders 48 for closing its gripping jaws 49 and 50. Each jackscrew 47 is mounted at its upper end for rocking about a horizontal pivot 51. The link 46 is driven by a piston and cylinder assemblages 52, the cylinder of which are mounted on trunnions 53 on the link 46 and the piston rods 54 of which are connected by pivots 55 to the brackets or plates 43 on the baseplate 13. Thus, the link 46, together with the stretch head 45, may be swung upwardly to loading position, and downwardly to final position in which the stock is wrapped entirely about the male die in the same manner as the link 24 and head 17 heretofore described.

It is desirable, however, that the coaxial pivots 51 be adjustable to different positions vertically and horizontally, and for this purpose the front and rear frame members 14 are provided at the upper end at the front and rear with rigid links 60, respectively, of which the upper ends are pivotally mounted on the pivots 31. The lower end of the links 60 support the pivots 51. The frame members 14 have rows of aligned passages 62 therein and each of the links 60 has a passage 63 of like size therethrough. The links 60 may be swung about the axis of the pivots 31 to different adjusted positions in which the passages 63 of the links are in alignment with one set of the aligned passages 62 of the support members 14. Locking pins 64 are then inserted to lock the links 60 in a fixed rotated position about the axis of the pivot 31, thus adjusting the position ofthe link pivot 51.

The die 5 is positioned with its crest at the axis of the pivot 31 as near as may be. A substantial adjustment is permitted by adjusting the position of the links 60. Since the male die can be adjusted to different positions endwise of the baseplate 13, the pivots 51 are thus adjustable bodily universally transversely of its axis relative to the male die to positions parallel to its starting position. Furthermore, since each of the links 24 and 46 is adjustable by means of its associated jackscrew portions, the heads 17 and 45 can be arranged in substantially any preselected relation in starting position and finishing position with respect to the die 5. The links 24 and 46 and associated mechanisms are duplicated at the rear of the structure, the front and rear links 24 being rigidly connected together by the head 17 and the front and rear links 46 being rigidly connected together by the head 45.

The movable gripper jaw carrier, such as 22, and the corresponding movable jaw carrier for the head 45, are preferably continuous and uninterrupted from the front to the rear of the fixture and press.

The cylinders 19 are connected in parallel with each other as also are cylinders 48 so that they act as a unit in applying the clamping pressure to the end margins of the sheet.

Thus, by proper operation of the piston and cylinder assemblages 35 and 52, the heads 17 and 45 may be swung to loading position, the sheet inserted and gripped by operation of the piston and cylinder assemblages 19 and 48, respecti ely, with sufficient gripping force so that, as the links 24 and 46 are swung downwardly to wrap the stock about the male die 5, the heads spread apart and first tension the stock into the yield range, preferably just as the stock has engaged the crest of the male die 5. They continue this clamping pressure and tension independently of die closure into a yield range until the heads reach their lowermost positions at which the stock has been wrapped about the male die. While they remain in this position, the ram descends to effect cooperation between the female die and the male die and thus to impart the final character lines and shape to the already substantially formed sheet. As mentioned, it often happens, especially when the male die has a concavity such as 5a and the female die has a protuberance such as 6a, which must press the metal of the sheet into this concavity, that a greater length of metal is required at this location in order to prevent overstressing of 5 the metal should the distance along the concavity be greater than the distance along the adjacent portions of the die. In order to eliminate such necessity, provision is made for reducing the gripping pressure of the jaws 18 and 23 or jaws 49 and 50, or both, depending upon the exigencies of the particular shape or workpiece. This reduction in clamping pressure is such as to relieve the independent tensioning of the sheet by the gripping heads and to allow the metal to slip and feed in between the dies as demanded by the closure and shape of the dies. The clamping pressure is reduced to an extent that the independent tension applied by the gripping heads, plus any tension imparted by the closure of the die, does not exceed the yield range of the stock. Usually, the independent tensioning force is reduced to zero so that the only stretch during final closure is imposed by the dies only.

This is possible because at the time the stock has been wrapped completely onto the male die, the die closure follows in such a short instant thereafter that the metal is still in its more readily deformable condition imparted by the independent stretching by the stretch heads, and has not work hardened as a result of the independent tensioning. Therefore, it can draw in to the extent necessary, with the tensioning requirement reduced by the feed in and slippage through the heads.

This is considerably different from the die drawing with hold down rings wherein the dies and hold down ring must retainthe stock in the yield range during closure in order for the stock to retain substantially the form of the cavity defined by the closed dies.

As one means for effecting this operation, a hydraulic system such as illustrated in FIGS. 5 and 6 is employed.

F IG.'5 shows the condition of the hydraulic system when the stretch heads are raised and held in position for loading, and FIG. 6 shows the system when the stretch heads are operated for tensioning the stock and wrapping it on the male die. In both FIGS. 5 and 6 the pressure side of the circuit is indicated by heavy lines and the return side by light lines for facilitating the tracing thereof.

Further, for convenience, only the front left head and cylinders are illustrated, it being understood that the front and rear cylinders for each stretch head are connected in parallel.

A similar circuit may be used for each of the right and left end fixtures, they being connected in parallel to the pump and to the sump, each through its own control valve.

Referring to FIG. 5, the hydraulic circuit, as applied to the left-hand unit of FIG. 3, is illustrated. The pressure fluid is supplied by a variable delivery pump P driven by a motor M,

the inlet of the pump being connected to a Sump S and the pressure side of the pump being connected by-a line 65 to a solenoid operated reversing valve 66. With the valve set in the position illustrated in FIG..5, pressure fluid is fed through a line 67 to the rod end of the cylinders 19 so as to open the clamping jaws. A branch line 68 from the line 67 leads to the discharge side of a sequence valve 69. A second branch line 70 leads from the line 67 through a check valve 71 to the inlet line 72 of the sequence valve 69. The line 70 also leads to a branch line 73 by which it is connected to the cylinder 35 at the side of the piston 39 for driving the piston on the extending stroke and thus lifting the head 17 to loading position. A pilot line 74 leads from the juncture of the line 70 and line 73 to a pressure reducing valve 75. A return line 78 is connected through thevalve 66 to the sump and to the inlet 79 of the pressure reducing valve 75. A branch line 80, having a check valve 81 to prevent the flow of pressure fluid therepast when the line 78 is connected to the pressure side of the pump, leads to the outlet 81 of the pressure reducing valve 75, and is connected by a lateral or branch line 82 to the head end of the cylinders 19. A pilot line 83 leads from the head end of the cylinders 19 to the sequence valve 69. Thus, in the position illustrated in FIG. 5, both piston rods 40 and 54 are in their fully extended positions, the gripping heads 17 and 45 are raised, and the clamping jaws are fully open for loading, and so remain as long as the reversing valve 66 is set in the position illustrated in FIG. 5.

Upon shifting of the reversing valve to the position illustrated in FIG. 6, pressure is supplied to the head ends of the clamp cylinders 19 through the pressure reducing valve 75 and directly to the right-hand end of the pulldown cylinders 35 which are connected in parallel with the input side of the valve 75 through the line 78. Concurrently the rod ends of the clamp cylinders 19 are vented to the sump S through the line 67 and valve 66. Pressure then builds up in the head ends of the clamp cylinders 19 and, by way of the pilot line 83, opens the sequence valve 69, thereby venting to sump S, with restricted flow, through lines 73, 72, 68, and 67, the left ends of the pulldown cylinder 35 so as to start the pulldown. The sequence valve 69 imposes back pressure in the left ends of the pulldown cylinder 35 and in the pilot line 74, which back pressure continues until the pulldown cylinders reach the lower end of their strokes, whereupon the flow in the pilot line 74 ceases and the back pressure drops to zero through the sequence valve 69.

Thus, as the pulldown cylinders are lowering, the reducing valve 75 is held to open at a maximum clamping pressure equal to the back pressure built up in the pilot line 74 and the setting of the spring 75a of the pressure reducing valve 75.

When, however, the pulldown cylinder stops and the flow ceases, back pressure in the left end thereof is vented through the lines 73, 70 and 72 and through the sequence valve 69 which remains open due to high pressure in the head end of the cylinder 19. When the pressure in the pilot line 74 drops to zero, then the pressure reducing valve 75 can unload at a pressure determined by setting of the spring 750, this unloading being through the pilot line 74. This reduces the pressure in the head end of the clamp cylinders 19.

For example, during operation, the pressure side of the system may be 1,000 p.s.i., this being pressure for the pulldown and for clamping. However, when the pulldown ceases and just before the female die closes, the clamping pressure may be reduced to as little as 75 to psi, thus terminating the independent tensioning by the heads and allowing the metal to slip from between the jaws 18 and 23. The pulldown cylinders reach the bottom of their stroke before closure of the female die, and before any amount of pull out of stock from between the jaws by the female die is necessary.

If in certain instances it is not desired to allow the stock to slip out of the jaws, then the pressure reduction valve can be set to maximum.

With the structure and circuit described, very precise control can be obtained as to the reduction of clamping pressure on the margins of the sheet in relation to the position and demands of the dies and so that the sheet can be formedwithout appreciable spring back while allowing some metal to be drawn in between thedies where indicated without over stretching any part of the sheet.

Having thus described my invention, I claim:

1. In a stretch draw apparatus,

a male drawing die,

a complementary female drawing die;

power means forclosing the dies;

stretch heads arranged at opposite limits of the dies;

supporting means supporting the heads for movement relative to each other for tensioning a sheet of metal into its yield range, independently of die closure, unidimensionally in the direction from one of its margins to its opposite margin, and to final fixed positions, respectively, relative to the male die for wrapping the sheet onto the male die while maintaining the sheet tensioned in said range;

pairs of gripping jaws carried by the heads, respectively, and

adapted to receive said opposite margins of the sheet of metal therebetween, respectively, and to be moved into clamping engagement therewith;

stretch applying power means for effecting said relative movement of the heads;

jaw-operating power means on each head for driving the pair of jaws of the associated head into clamping relation to the adjacent sheet margin with sufficient clamping pressure to hold the margin in substantially fixed relation to the associated jaws during the independent tensioning of the sheet into the yield range by said movements of the heads and until the heads reach said final positions, respectively;

control means for the jaw-operating power means and operable to cause the jaw-operating power means to maintain said clamping pressure until the heads complete their respective wrapping movements and reach said final positions, and operable thereafter while the heads remain in their final positions and the dies are yet unclosed, to cause the jaw-operating power means to reduce the clamping pressure to a degree such that, during closure of the dies, the margins can slip relative to, and against frictional resistance imposed by, the jaws sufficiently to substantially eliminate the maintenance of the independent tensioning by the heads and to permit the respective sheet portions between each margin and the dies to be drawn into the dies against said frictional resistance by forces imposed on the sheet by the dies, while the jaws hold the sheet steady in the dies.

2. The structure according to claim 1 wherein each pair of jaws includes a movable jaw, the jaw-operating power means for each pair of jaws comprise piston and cylinder assemblages carried by the stretch head associated with such pair, and each assemblage includes a cylinder member and a piston member, one of said members is connected to the associated head and the other is directly connected to its associated movable jaw so as to apply the thrust of the assemblage to the movable jaw.

3. The structure according to claim 1 wherein the control means are rendered operative to cause the jaw-operating power means to effect said reduction in said clamping pressure consequent upon the stretch heads reaching said final positions.

4. The structure according to claim 1 wherein the male die has a crest extending generally transversely thereof,

the supporting means of each head comprises a rigid linkage,

pivot means support the linkage for swinging about a predetermined axis, and

the pivot means for the linkage of one head is positioned with its axis extending in a direction transversely of the dies and close to said crest of the male die.

5. In a stretch draw apparatus;

a male drawing die;

a complementary female drawing die;

power means for closing the dies;

stretch heads arranged at opposite ends of the male die and adapted to grip opposite margins of a sheet of metal, respectively;

supporting means supporting the heads for swinging movement relatively toward and away from each other, respectively, endwise of the male die for tensioning the sheet into its yield range, independently of die closure, unidimensionally in the direction from one of said margins to the other, and to final fixed positions, respectively, relative to the fonning face of the male die for wrapping the sheet onto the male die while maintaining the sheet tensioned in said range;

pairs of gripping jaws carried by the heads, respectively;

said supporting means of each head including a rigid link to which the head is connected for swinging therewith; and

pivot means supporting each of said links for swinging toward and away from the adjacent end of the male die about a predeterrnine axis spaced from the head, and closer to the longitudinal midportion of the male die than is the head in the tensioning positions of the head; means supporting the pivot means of at least one of said links for bodily movement, relative to and endwise of, the male die, transversely of its axis to selected adjusted parallel positions and operable to hold the pivot means of said one of said links fixedly in the selected adjusted position during said tensioning of the sheet; and

stretch power means for swinging the links away from the ends of the male die and thereby causing the heads to apply said tensioning while the means supporting the pivot means remain fixed in said selected adjusted position.

6. A stretch unit according to claim 5 wherein said means which support the pivot means of said one of said links is mounted on the frame for swinging movement about a second axis parallel to, and offset from, said predetermined axis to selected adjusted positions; and

means are provided which secure said support means of said one of said links in said adjusted positions about said second axis, respectively.

7. A method of stretch drawing sheet metal stock between complementary male and female drawing dies, comprising:

applying force to two opposite margins of a sheet to a degree sufficient to tension the sheet unidimensionally into its yield range and to elongate the sheet along said dimension independently of the dies; then, while maintaining the said independent force at said degree, moving said margins of the sheet facewise of the sheet in a direction transversely of said tensioning dimension to thereby stretch wrap the sheet onto the male die;

discontinuing said movement of the margins when the margins are in the final stretch wrapping position;

moving the dies to closed position;

maintaining said independent force at said degree substantially until the initiation of the drawing of the sheet by cooperation of the dies;

then, while said margins remain in said final stretch wrapping position, reducing said applied force to a lesser degree such as to result in discontinuance of said independent tensioning into the yield range and to a degree to hold the sheet steady in position on the male die while permitting the margins to move toward the dies under relatively low resistance so that the material between the margins and the dies can be drawn into the space betwee.: the dies by the dies themselves; and

maintaining said force at said lesser degree until the final closure of the dies.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,597,955 Dated August 1% 1971 Inventofls) Robert A. Mackenzie It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Page 1, reference to Mackenzie should read "3,452,573". Col. 2, line 1 1-, (second occurrence) "the" should read --that--; Col. 3, line 54, "a out" should read "about--.

Signed and sealed this 7th day of March 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents TM PO-IOSO (10-69) USCOMM-DC 60375-969 0 u s covzmmzm PRINTING OFFICE: 1959 o-see-au 

1. In a stretch draw apparatus, a male drawing die, a complementary female drawing die; power means for closing the dies; stretch heads arranged at opposite limits of the dies; supporting means supporting the heads for movement relative to each other for tensioning a sheet of metal into its yield range, independently of die closure, unidimensionally in the direction from one of its margins to its opposite margin, and to final fixed positions, respectively, relative to the male die for wrapping the sheet onto the male die while maintaining the sheet tensioned in said range; pairs of gripping jaws carried by the heads, respectively, and adapted to receive said opposite margins of the sheet of metal therebetween, respectively, and to be moved into clamping engagement therewith; stretch applying power means for effecting said relative movement of the heads; jaw-operating power means on each head for driving the pair of jaws of the associated head into clamping relation to the adjacent sheet margin with sufficient clamping pressure to hold the margin in substantially fixed relation to the associated jaws during the independent tensioning of the sheet into the yield range by said movements of the heads and until the heads reach said final positions, respectively; control means for the jaw-operating power means and operable to cause the jaw-operating power means to maintain said clamping pressure until the heads complete their respective wrapping movements and reach said final positions, and operable thereafter while the heads remain in their final positions and the dies are yet unclosed, to cause the Jaw-operating power means to reduce the clamping pressure to a degree such that, during closure of the dies, the margins can slip relative to, and against frictional resistance imposed by, the jaws sufficiently to substantially eliminate the maintenance of the independent tensioning by the heads and to permit the respective sheet portions between each margin and the dies to be drawn into the dies against said frictional resistance by forces imposed on the sheet by the dies, while the jaws hold the sheet steady in the dies.
 2. The structure according to claim 1 wherein each pair of jaws includes a movable jaw, the jaw-operating power means for each pair of jaws comprise piston and cylinder assemblages carried by the stretch head associated with such pair, and each assemblage includes a cylinder member and a piston member, one of said members is connected to the associated head and the other is directly connected to its associated movable jaw so as to apply the thrust of the assemblage to the movable jaw.
 3. The structure according to claim 1 wherein the control means are rendered operative to cause the jaw-operating power means to effect said reduction in said clamping pressure consequent upon the stretch heads reaching said final positions.
 4. The structure according to claim 1 wherein the male die has a crest extending generally transversely thereof, the supporting means of each head comprises a rigid linkage, pivot means support the linkage for swinging about a predetermined axis, and the pivot means for the linkage of one head is positioned with its axis extending in a direction transversely of the dies and close to said crest of the male die.
 5. In a stretch draw apparatus; a male drawing die; a complementary female drawing die; power means for closing the dies; stretch heads arranged at opposite ends of the male die and adapted to grip opposite margins of a sheet of metal, respectively; supporting means supporting the heads for swinging movement relatively toward and away from each other, respectively, endwise of the male die for tensioning the sheet into its yield range, independently of die closure, unidimensionally in the direction from one of said margins to the other, and to final fixed positions, respectively, relative to the forming face of the male die for wrapping the sheet onto the male die while maintaining the sheet tensioned in said range; pairs of gripping jaws carried by the heads, respectively; said supporting means of each head including a rigid link to which the head is connected for swinging therewith; and pivot means supporting each of said links for swinging toward and away from the adjacent end of the male die about a predetermine axis spaced from the head, and closer to the longitudinal midportion of the male die than is the head in the tensioning positions of the head; means supporting the pivot means of at least one of said links for bodily movement, relative to and endwise of, the male die, transversely of its axis to selected adjusted parallel positions and operable to hold the pivot means of said one of said links fixedly in the selected adjusted position during said tensioning of the sheet; and stretch power means for swinging the links away from the ends of the male die and thereby causing the heads to apply said tensioning while the means supporting the pivot means remain fixed in said selected adjusted position.
 6. A stretch unit according to claim 5 wherein said means which support the pivot means of said one of said links is mounted on the frame for swinging movement about a second axis parallel to, and offset from, said predetermined axis to selected adjusted positions; and means are provided which secure said support means of said one of said links in said adjusted positions about said second axis, respectively.
 7. A method of stretch drawing sheet metal stock between complementary male and female drawing dies, comprising: APPLYING force to two opposite margins of a sheet to a degree sufficient to tension the sheet unidimensionally into its yield range and to elongate the sheet along said dimension independently of the dies; then, while maintaining the said independent force at said degree, moving said margins of the sheet facewise of the sheet in a direction transversely of said tensioning dimension to thereby stretch wrap the sheet onto the male die; discontinuing said movement of the margins when the margins are in the final stretch wrapping position; moving the dies to closed position; maintaining said independent force at said degree substantially until the initiation of the drawing of the sheet by cooperation of the dies; then, while said margins remain in said final stretch wrapping position, reducing said applied force to a lesser degree such as to result in discontinuance of said independent tensioning into the yield range and to a degree to hold the sheet steady in position on the male die while permitting the margins to move toward the dies under relatively low resistance so that the material between the margins and the dies can be drawn into the space between the dies by the dies themselves; and maintaining said force at said lesser degree until the final closure of the dies. 